The present invention relates to an apparatus and method for reducing the presence of noise in a digital video image. The invention provides adaptive pre-processing of image data prior to compression and transmission to enhance the quality of the received image. A low-cost implementation comprising finite impulse response (FIR) filters is also disclosed.
A video image is defined by a number of picture elements, also known as pixels or pels. A pixel, which is the smallest element of a raster scan line in the image, has an associated color space. For example, in a YCrCb color space which conforms to the CCIR 601 specification, Y is a luminance component, and Cr and Cb are color difference components. Y is defined to have a nominal range of 16 to 235, with Cr and Cb each having ranges from 16 to 240, with 128 indicating a zero color difference (e.g., the color white). Various sampling formats have been defined, including 4:4:4, 4:2:2, and 4:1:1. For example, with a 4:2:2 format, the Y component is sampled at twice the rate of the Cr and Cb components. With this format, the sampling frequencies for the Y, Cr and Cb components are 13.5 MHz, 6.75 MHz and 6.75 MHz, respectively. Typically each component comprises 8 data bits for standard video and computer applications, while high-end video applications, such as those conforming to the D1 standard, may use 10 bits per component. Thus, a single pixel may be defined by 24 or 30 bits. The YCrCb color space is a scaled version of the YUV color space commonly used with the Phase Alternation Line (PAL), National Television System Committee (NTSC), and the Sequentiel Coleur Avec Memoire (SECAM) standards. Various other color space standards exist.
Noise can be introduced into the pixel data prior to compression and transmission from various sources, including imaging equipment, recording equipment, channel noise in a channel over which the pixel data is communicated, and atmospheric factors. Moreover, noise in the pre-compressed video data hampers the performance of subsequent compression systems. Specifically, noise consumes data bits unnecessarily and deteriorates the visual quality of the resulting video image since compression processing, such as quantizing the coefficients of the Discrete Cosine Transformation or other spatial transformation, tends to increase the visibility of the noise. Thus, the presence of pixel noise can be amplified, leading to visible noise artifacts and other image degradation.
Compression encoding is typically performed on the pixel data prior to transmission to decrease the required bandwidth of the communication channel. Various video data compression standards are well known in the art, including the MPEG-2 standard, and the similar DigiCipher.RTM. II system, proprietary to the assignee hereof.
While filtering can reduce noise in a video image, it is desirable to avoid unnecessary filtering which can reduce the resolution (e.g., sharpness) of an image. Such over-filtering of pixel data reduces resolution by making it harder for the viewer to detect transitions, or edges, between image elements. Generally, an edge is a change in pixel amplitude, such as a color difference and/or luminance amplitude change, which may indicate the edge or outline of an object in an image. For example, a video image of a green field and a blue sky will exhibit an edge at the transition between the field and the sky. Similarly, a video image of a bright area and a shadowy area will exhibit an edge at the transition area. Accordingly, it has been problematic to preserve such true edges while reducing the effects of noise.
Accordingly, it would be desirable to provide a method and apparatus for reducing noise in a pre-compressed digital video signal. It would be desirable to provide a low-cost apparatus to filter each pixel in a video image by detecting the likelihood of the presence of an edge by computing the changes in pixel amplitude values on both sides of a current pixel in a frame, and/or by computing the changes in pixel amplitude values in video frames which precede and follow a current pixel frame. The system should effectively attenuate noise while minimizing any degradation in picture resolution. The present invention provides a system having the above and other advantages.